System for supplying fuel from a tank (2) to a manifold (5) that distributes fuel into an engine (3); the system (1) having a pumping unit which connects together the tank (2) and the manifold (5) and has a fluid line (11) with a first or low-pressure portion (P1), a second or high-pressure portion (P2), a high-pressure pump (9) connecting together the first and second portions (P1, P2); the first portion (P1) having a plurality of low-pressure branches (23-28, 30-34), a plurality of fluid components (8, 13, 21, 29, 231, 232, 233, 241, 242, 251, 261, 271, 311), each of which has at least one connector (36) and at least one tubular coupling (40; 140; 340; 440), which latter is inserted partly into the connector (36) and partly into a low-pressure branch (23-28; 30-34); each coupling (40; 140; 340; 440) having a plastic tubular body (145; 345; 445).